1. Field of the Invention
The present invention relates generally to a communication apparatus and method of a mobile communication system using Orthogonal Frequency-Division Multiple Access (OFDMA), and more particularly, to a method and apparatus for transmitting and receiving an uplink acknowledgement channel for a downlink data channel.
2. Description of the Related Art
In general, in a mobile communication system, Hybrid Automatic Repeat request (HARQ) is an important technology used to increase the reliability of data transmission and the data throughput in a packed-based mobile communication system. HARQ refers to a technology obtained by combining the Automatic Repeat Request (ARQ) technology with the Forward Error Correction (FEC) technology.
FIG. 1 is a diagram illustrating an example of a typical HARQ. In FIG. 1, the horizontal axis is a time axis.
Referring to FIG. 1, in a mobile communication system, a base station transmits a plurality of data channels, and a terminal receives data by demodulating the data channels. Here, the base station can repeatedly transmit the same data or consecutively transmit different data through the data channels.
The base station performs an initial transmission operation 101 for a specific data channel. The terminal apparatus that receives the initial transmission data attempts to demodulate the data channel. In this process, the terminal performs a Cyclic Redundancy Check (CRC) for the data channel. If, as a result of the check, the initial transmission data is determined not to have been successfully demodulated, the terminal feedbacks Non-Acknowledgement (NACK) 102 to the base station. When the NACK 102 is received, the base station performs a first retransmission operation 103, as a retransmission for the initial transmission operation 101. Thus, the data channels in the initial transmission operation 101 and the first retransmission operation 103 transmit the same data. It should be noted that, although the data channels transmit the same data, they may include different redundancies.
It is assumed that the data transmission operations for the same information, that is, each of the transmission operations 101, 103, 105, or 107 for the same information is called a sub-packet. The terminal that receives the first retransmission 103 combines the received retransmitted data and the initially transmitted date the received in the initial transmission 101 in accordance with a predetermined rule, and attempts to demodulate the data channels based on the combined result.
When the data transmission is not successfully demodulated through CRC for the data channels in the above process, the terminal feedbacks a NACK 104 to the base station. The base station receives the NACK 104, and then performs a second retransmission operation 105, after a lapse of a given time from the first retransmission operation 103.
Thus, all the data channels for the initial transmission operation 101, the first retransmission operation 103, and the second retransmission operation 105 transmit the same information.
After the terminal receives the second retransmission data, the terminal performs combining with respect to the initial transmission data, the first retransmission data, and the second retransmission data in accordance with a predetermined rule, and demodulates the data channels based on the combining result.
As illustrated in FIG. 1, it is assumed that the data is successfully demodulated through CRC for the data channels, after the second retransmission 105.
Accordingly, after successful demodulation, the terminal feedbacks an ACK 106 to the data base station. The base station receives the ACK 106, and performs an initial transmission 107 for a sub-packet of a next data information. The initial transmission operation 107 can be performed immediately after the ACK 106 is received, or after a given time has elapsed, which results from a predetermined scheduling result.
As described above, in order to support HARQ, a terminal should transmit ACK/NACK feedbacks. A channel that transmits the ACK/NACK is called a response channel, or ACK CHannel (ACKCH).
FIG. 2 is a diagram illustrating a Physical Downlink Control CHannel (PDCCH) and a Physical Uplink Control CHannel (PUCCH) of a conventional mobile communication system.
Referring to FIG. 2, a base station configures PDCCHs 202 to 204 using at least one Control Channel Element (CCE) 201 to transmit. Here, a single PDCCH can use one, two, four, or eight CCEs 201. As illustrated in FIG. 2, each of the PDCCHs 202 to 204 can be used as a DownLink (DL) Grant. That is, the PDCCHs 202 to 204 can be used to allocate the resources of a Physical Downlink Shared CHannel (PDSCH) (i.e., a downlink data channel).
A terminal assigned with the resources of the PDSCH through the PDCCHs 202 to 204 transmits ACK/NACK information through channel resources for an ACKCH 206, which is mapped to the first CCE 201 of each PDCCH with respect to data transmitted through the allocated PDSCH resources. As an example of the above method, an ACKCH 1 is composed of channel resources called an ACKCH R1. Here, the channel resources refer to a Cyclic Shift (CS) and an Orthogonal Cover (OC) for configuring the ACKCH 206. The terminal can determine a CS and an OC, which are the resources of the ACKCH 206, using a channel index mapped to the CCE 201, as shown in Table 1 below.
TABLE 1Resource allocation: 18ACK/NACK channels with normal CPCELL SPECIFICACK/NACKCYCLIC SHIFTRS ORTHOGONALORTHOGONALOFF SETCOVERCOVERδoffsetPUCCH = 1δoffsetPUCCH = 0 noc = 0 noc = 1 noc = 2noc = 0noc = 1noc = 2ncs = 1ncs = 0n′ = 012n′ = 01221663211311343775421421465887631531587999841641610 9101011 105175170111111
Table 1 illustrates configurations of 18 ACKCHs in order to minimize interference of each sequence when a normal Cyclic Prefix (CP) is used. For example, when a channel index mapped to the CCE 201 is 5 and a preset δoffsetPUCCH is 0, the terminal generates the ACKCH 206 using CS=10 and OC=0.
FIG. 3 is a diagram illustrating an ACKCH configuration using a CS and an OC, which are resources of an ACKCH corresponding to each index.
Referring to FIG. 3, as to the ACK/NACK information, a Computer-Generated (CG) sequence is generated using an allocated CS. The generated sequence is copied into four sequences. Each of the four copied sequences passes through Inverse Fast Fourier Transforms (IFFTs) 301, is multiplied by one bit of an OC by multipliers 302, and is then mapped 303 to each symbol of resource block allocated to a PUCCH. The mapped ACK/NACK symbol, together with uplink reference signal symbols, is hopped to each of slots within one sub-frame through one antenna, so that the same seven symbols are mapped.
The PDCCH includes one or several CCEs in order to transmit information having a different length according to the property of a DL grant and to increase the reliability of PDCCH depending on channel conditions by using a different number of CCEs according to channel conditions. However, assuming that channel resources are allocated to CCEs of PDCCH, respectively, when uplink ACK/NACK is transmitted, the channel resources should be allocated with the same number as CCEs. However, when the PDCCH includes several CCEs, the efficiency of resources decreases because only the first channel resource among uplink channel resources mapped to the CCEs is used and the remaining channel resources are not used.